Fuel assembly and thimble screw of the same

ABSTRACT

A fuel assembly includes a bottom nozzle set on a lower core plate of a nuclear reactor, a top nozzle with a hold down spring to urge the bottom nozzle against the lower core plate, guide thimbles which guide control rods, having passed through the top nozzle, toward the lower core plate, a dashpot formed on each of the guide thimbles to reduce the fall velocity of a corresponding one of the control rods, a thimble screw which connects each of the guide thimbles to the bottom nozzle, and a drain hole formed to extend through each of the thimble screw. The dashpot has a large-diameter portion with substantially the same diameter as that of each of the guide thimbles. The diameter d of the drain hole falls within a range of 0.04D&lt;d&lt;0.08D where D is an inner diameter of the large-diameter portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2001-107148, filed Apr. 5,2001, No. 2001-129035, filed Apr. 26, 2001; and No. 2002-013333, filedJan. 22, 2002, the entire contents of all of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel assembly used in a nuclearreactor, and a thimble screw of the fuel assembly.

2. Description of the Related Art

An example of a nuclear reactor currently widely used for powergeneration includes a pressurized water reactor (to be referred to as“PWR” hereinafter). A fuel assembly used by the PWR is generally acanless fuel assembly with no wrapper tube. The structure of the canlessfuel assembly will be briefly described. Top and bottom nozzles eachhaving a plurality of coolant flow holes are connected to each otherwith a plurality of control rod guide tubes extending parallel to eachother.

More specifically, the upper ends of the control rod guide tubes, i.e.,so-called guide thimbles, are mechanically connected to the top nozzle,and the lower ends thereof are also mechanically connected to the bottomnozzle. These guide thimbles respectively accept the thin elongatedcontrol rods of a control rod cluster. Depending on the loading positionof the fuel assembly in the core, the guide thimbles do not accept thecontrol rods as they are not located at corresponding positions. In thiscase, the guide thimbles accept non fuel bearing components (NFBC) suchas thimble plugs or burnable poisons. A plurality of grids are mountedon the guide thimbles. The fuel rods are accepted in the latticeopenings and are elastically supported there.

Of the structure of the fuel assembly briefly described above, thestructure of the connecting portion that connects the guide thimbles andbottom nozzle will be described in more detail with reference to theaccompanying drawings.

FIG. 1 is an elevation schematically showing the structure of a fuelassembly applied to a PWR.

FIG. 2A is a sectional elevation showing part of the lower structure ofthis fuel assembly, and FIG. 2B is a bottom view of the same.

As shown in FIG. 1, a fuel assembly 1 has top and bottom nozzles 3 and 4at upper and lower ends of elongated guide thimbles 2, and a top grid 5,middle grids 6, and bottom grid 7 fixed to the guide thimbles 2 in thelongitudinal direction. Each of the top and bottom grids 5 and 7 isformed from a large number of lattice frames using thin plates, andholds fuel rods 8.

The top nozzle 3 is a bottomed box-like structure with a substantiallysquare horizontal section. The top nozzle 3 has a plurality of coolantflow holes and guide thimble mounting holes in its end platecorresponding to the bottom plate. In addition, a hold down spring 9 isattached to the upper portion of the top nozzle 3. The bottom nozzle 4has a top or end plate with a substantially square shape when seen fromabove, where a plurality of coolant flow holes and guide thimblemounting holes are formed. Legs 10 are respectively integrally formed toproject from the four corners of the lower surface of the end plate.

The top and bottom nozzles 3 and 4 are connected to the upper and lowerends of the plurality of hollow tube-like guide thimbles 2 by utilizingthe mounting holes described above.

Referring to FIGS. 2A and 2B, the lower end of each hollow tube-likeguide thimble 2 is welded to a thimble end plug 12, and is fixed to thebottom nozzle 4 with a thimble screw 14 through an insert 13. One topgrid 5 and seven middle grids 6 are mounted on the guide thimbles 2 atintervals, and the bottom grid 7 is mounted on the guide thimbles 2through its connecting structure. It should be understood that thenumber of middle grids 6 can be appropriately changed.

The bottom grid 7 is fixed to the upper portion of the insert 13. Adrain hole 15 extends through the thimble screw 14 in the axialdirection, and a rotation preventive pin 17 for preventing loosening ofthe thimble screw 14 is provided to a seat 16 of the drain hole 15. Thedrain hole 15 allows the coolant in use to flow in the core in adirection P shown in FIG. 2A.

Furthermore, the seat 16 has a spot facing hole 18 communicating withthe lower portion of the drain hole 15 and reaching the bottom surfaceof the seat 16. The rotation preventive pin 17 does not interfere withthe flow of the coolant flowing into the drain hole 15 in the directionP.

The fuel rods 8 are inserted in and supported by the aligned latticeopenings of the upper, middle, and bottom grids 5, 6, and 7 one by one,thus forming the fuel assembly 1.

With this structure, the drain holes 15 of the thimble screws 14 guidethe coolant into the guide thimbles 2 in the core, and the introducedcoolant cools the non fuel bearing components mounted in the guidethimbles 2. The drain holes 15 also serve as holes for sending the innercoolant to the outside.

During a scram mode of the nuclear reactor, the control rods areurgently inserted in the guide thimbles 2 by free fall. The drain holes15 also serve as a restrictor for limiting the outlow velocity of theinner coolant so the fall impact is moderated. In other words, to assurethe cooling function described above, the larger the diameter d of thedrain hole 15 of the thimble screw 14, the better. To moderate the fallimpact produced when the control rods fall, the smaller the diameter d,the better, which is contradictory.

During the scram mode of the nuclear reactor, when the control rods areurgently inserted in the guide thimbles 2 by free fall, an excessivelylarge impact occurs to the top nozzle 3. For this reason, the guidethimbles 2 respectively have thin tube-like dashpots 20. The dashpots 20reduce the velocity of the control rods falling in the guide thimbles 2,thereby moderating the excessively large impact acting on the top nozzle3.

According to an example of the fuel assembly 1 with such dashpots 20, asshown in FIG. 3, a dashpot 20 with a length of 0.16L to 0.18L isprovided to the guide thimble 2 where L is the length of the guidethimble 2 along its axial direction. Therefore, the compression loadacting on the guide thimble 2 in the axial direction may cause flexuraldeformation of the dashpot 20. In this case, the control rod may not beinserted well.

For this reason, as shown in FIGS. 4 and 5, a technique is disclosed inwhich the length of the dashpot 20 of the guide thimble 2 is decreased.With this arrangement, the length of the dashpot 20 with respect to thelength L of the guide thimble 2 can be suppressed to fall within therange of 0.03L to 0.1L, so the flexural rigidity of the dashpot 20 isincreased. This can prevent flexural deformation of the dashpot 20.

This guide thimble will be referred to as an improved guide thimblehereinafter. The lower structure of a fuel assembly 1 to which animproved guide thimble shown in FIG. 5 is applied is different from thatof the fuel assembly 1 shown in FIG. 2A only in that sleeves 21 areprovided at the bottom grids 7 and that it has a dashpot 20 only at oneportion, and is substantially the same as that of the fuel assemblyshown in FIG. 2A.

In the fuel assembly to which the improved guide thimble is applied, thelength of the dashpot 20 on the lower end side of the guide thimbles 2is decreased, as shown in FIGS. 4 and 5. This increases the flexuralrigidity of the dashpot 20 to prevent its flexural deformation. However,a so-called braking effect that moderates the fall velocity of thecontrol rod is decreased.

In a PWR, its fall terminal velocity is limited from the viewpoint ofensuring the safety of the fuel assembly 1. Originally, the dashpot 20is provided to the guide thimble 2 in an axial direction, as shown inFIG. 3, in order to moderate the fall velocity of the control rod suchthat the fall terminal velocity does not exceed a limit. For thisreason, in the fuel assembly 1 employing the improved guide thimble asshown in FIGS. 4 and 5, a countermeasure that moderates the fallterminal velocity of the control rod must be provided by another means.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the above situation, andhas as its object to provide a fuel assembly in which an improved guidethimble is employed and the diameter of the drain hole of a thimblescrew is adjusted so that a fall impact produced when a control rodfalls is moderated, and flexural deformation of a dashpot is prevented,without impairing the cooling function of non fuel bearing components,and a thimble screw of the fuel assembly.

In order to achieve the above object, the present invention has thefollowing means.

According to a first aspect of the present invention, there is provideda fuel assembly comprising a bottom nozzle set on a lower core plate ofa nuclear reactor, a top nozzle with a hold down spring to urge thebottom nozzle against the lower core plate, a plurality of control rodguide tubes which guide control rods, having passed through the topnozzle, toward the lower core plate, top, middle, and bottom gridsmounted on the control rod guide tubes, a plurality of fuel rods held bythe grids to be substantially parallel to the control rod guide tubes, athin tube-like dashpot formed on each of the control rod guide tubes toreduce a fall velocity of a corresponding one of the control rods, athimble screw which connects each of the control rod guide tubes to thebottom nozzle, and a drain hole formed to extend through the thimblescrew. The dashpot has a large-diameter portion, at a lower portionthereof, with substantially the same diameter as that of each of thecontrol rod guide tubes, and a diameter d of the drain hole falls withina range of 0.04D<d<0.08D where D is an inner diameter of thelarge-diameter portion.

Hence, the coolant can be sufficiently supplied also from the viewpointof assuring the cooling function of the non fuel bearing components.From the viewpoint of moderating the fall impact of the control rod aswell, the terminal velocity of the control rod can be suppressed to beequal to or less than the fall velocity with which the fall impact ofthe control rod can be moderated. Therefore, flexural deformation of thedashpot can be prevented.

A fuel assembly according to a second aspect of the present inventionhas the following thimble screw.

More specifically, the thimble screw is disposed in a bottom nozzle soas to extend from a bottom surface side, has a drain hole extending inthe thimble screw in a longitudinal direction from a spot facing hole ofa seat to a distal end, is formed such that a coolant is supplied intothe drain hole from the spot facing hole toward the distal end while thenuclear reactor operates and into the drain hole from a distal end sidetoward the spot facing hole during a scram mode, is locked to the bottomnozzle at the seat with a rotation preventive pin, and is provided witha coolant collision portion, at a drain hole side of the rotationpreventive pin, against which the coolant flowing from the distal endside toward the spot facing hole collides in order to increase thepressure drop of the coolant during the scram mode.

Furthermore, a collision surface of this coolant collision portionagainst which the coolant collides forms a recessed surface ground in aV-shape from the distal end side toward the spot facing hole, a flatsurface, or a recessed surface arcuately ground from the distal end sidetoward the spot facing hole.

With the above arrangement, the thimble screw can also serve as a diode.Thus, while the non fuel bearing components have the same coolingability as that of the conventional case, the decelerating effect of thecontrol rod can be improved by the increase in fluid resistance.

A fuel assembly according to a third aspect of the present invention hasthe following thimble screw.

More specifically, the thimble screw is disposed in a bottom nozzle toextend from a bottom surface side, has first and second drain holesextending in the thimble screw in a longitudinal direction from a spotfacing hole of a seat to a distal end, and is formed such that a coolantis supplied into the first and second drain holes from the spot facinghole toward a distal end side while the nuclear reactor operates andinto the second and first drain holes from the distal end side towardthe spot facing hole during a scram mode. The thimble screws is lockedto the bottom nozzle at the seat with a rotation preventive pin. Thefirst drain hole has an opening area smaller than an opening area of thespot facing hole and smaller than an opening area of the second drainhole at the distal end.

Furthermore, the thimble screw is provided with a coolant collisionportion, at a first drain hole side of the rotation preventive pin,against which the coolant flowing from the distal end side toward thespot facing hole collides in order to increase the pressure drop of thecoolant during the scram mode.

The collision surface of the coolant collision portion against which thecoolant collides forms a recessed surface ground in a V-shape from thedistal end side toward the spot facing hole, a flat surface, or arecessed surface arcuately ground from the distal end side toward thespot facing hole.

Since the thimble screw has the above arrangement, the coolant entersfrom the distal end side and is discharged in the form of a jet from thedrain hole toward the rotation preventive pin. As the rotationpreventive pin strongly functions as a fluid resistance, the pressuredrop for the flow of the coolant in the scram mode can be increased, andthe decelerating effect of the control rod can be improved. Meanwhile,the rotation preventive pin does not influence the flow rate resistanceof the coolant when the nuclear reactor operates. Thus, the coolant flowrate is assured, and the cooling ability of the non fuel bearingcomponents can maintain the same effect as that of the conventionalcase.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is an elevation briefly showing the structure of a fuel assemblyto be applied to a PWR;

FIG. 2A is a sectional elevation showing part of the lower structure ofthe fuel assembly to be applied to the PWR;

FIG. 2B is a bottom view of the fuel assembly to be applied to the PWR;

FIG. 3 is an elevation of a guide thimble with dashpots at two portions;

FIG. 4 is an elevation showing part of the lower structure of a fuelassembly to which an improved guide thimble is applied;

FIG. 5 is an elevation showing part of the lower structure of a fuelassembly to which an improved guide thimble is applied;

FIG. 6 is a graph showing the relationship between ((drain hole diameterd of thimble screw)/(inner diameter D of lower large-diameter portion ofguide thimble)) and ((fall terminal velocity V of control rod)/(limitedfall terminal velocity V₀ of control rod));

FIG. 7 is a graph showing the relationship between ((drain hole diameterd of thimble screw)/(inner diameter D of lower large-diameter portion ofguide thimble)) and ((coolant inflow amount C from thimblescrew)/(coolant inflow amount C₀ from thimble screw which is necessaryfor cooling non fuel bearing components));

FIG. 8 is a view showing a state wherein a rotation preventive pin for athimble screw in a fuel assembly according to the second embodiment ofthe present invention is built into the thimble screw;

FIG. 9A is a view showing an example of the rotation preventive pin forthe thimble screw in the fuel assembly according to the secondembodiment of the present invention;

FIG. 9B is a view showing another example of the rotation preventive pinfor the thimble screw in the fuel assembly according to the secondembodiment of the present invention;

FIG. 9C is a view showing still another example of the rotationpreventive pin for the thimble screw in the fuel assembly according tothe second embodiment of the present invention;

FIG. 10 is a view showing an example of a thimble screw in a fuelassembly according to the third embodiment of the present invention; and

FIG. 11 is a view showing another example of a thimble screw in a fuelassembly according to the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be described hereinafterwith reference to the accompanying drawings.

Regarding the reference numerals used to describe the followingembodiments, the same reference numerals denote the same portions asthose of FIGS. 1 to 5.

First Embodiment

The first embodiment of the present invention will be described withreference to FIGS. 6 and 7.

In a fuel assembly according the first embodiment of the presentinvention, the structure of a connecting portion for connecting itsguide thimbles 2 and bottom nozzle 4 is as shown in FIGS. 2B and 4 or 5,and a so-called improved guide thimble is employed. An inner diameter Dof the lower large-diameter portion of the guide thimble 2 and adiameter d of a drain hole 15 satisfy the following equation (1):0.04D<d<0.08D  (1)

The function of the fuel assembly according to this embodiment with theabove arrangement will be described.

FIG. 6 is a graph showing results obtained by measuring a terminalvelocity V of a control rod inserted in the guide thimble 2 by free fallin a fuel assembly 1 formed as shown in FIGS. 4 and 5, by using (d/D),which is the ratio of the diameter d of the drain hole 15 of a thimblescrew 14 to the inner diameter D of the lower large-diameter portion ofthe guide thimble 2, as a parameter.

The axis of ordinate indicates V/V₀ obtained by dividing the terminalvelocity V of the control rod inserted in the guide thimble 2 by freefall by a limited terminal velocity V₀ determined from the viewpoint ofmoderating the fall impact of the control rod. More specifically, therange of (V/V₀)<1 is a range where the terminal velocity V of thecontrol rod inserted in the guide thimble 2 by free fall can besuppressed to be lower than the limited terminal velocity V₀. The rangeof (V/V₀)≧1 is a range where the terminal velocity V of the control rodinserted in the guide thimble 2 by free fall becomes equal to or morethan the limited terminal velocity V₀.

As shown in FIG. 6, in the range of (d/D)<0.08, (V/V₀)<1 is established,and the terminal velocity V of the control rod inserted in the guidethimble 2 by free fall does not exceed the limited terminal velocity V₀but satisfies the design standard. In the range of (d/D)≧0.08, (V/V₀)≧1is established, and the terminal velocity V of the control rod insertedin the guide thimble 2 by free fall exceeds the limited terminalvelocity V₀ and does not satisfy the design standard.

Hence, from the viewpoint of the terminal velocity V of the control rodinserted in the guide thimble 2 by free fall, the diameter d of thedrain hole 15 of the thimble screw 14 and the inner diameter D of thelower large-diameter portion of the guide thimble 2 must satisfyd<0.08D.

As described earlier, the drain hole 15 of the thimble screw 14 servesto guide the coolant into the guide thimble 2 in order to cool the nonfuel bearing components. From this viewpoint of assuring the coolingfunction, the larger the diameter d of the drain hole 15 of the thimblescrew 14, the better.

FIG. 7 is a graph showing results obtained by measuring the coolingability of the non fuel bearing components in a fuel assembly 1 formedas shown in FIGS. 4 and 5, by using (d/D), which is the ratio of thediameter d of the drain hole 15 of a thimble screw 14 to the innerdiameter D of the lower large-diameter portion of the guide thimble 2,as a parameter.

The axis of ordinate indicates C/C₀ obtained by dividing a coolantinflow amount C from the thimble screw 14 by a coolant inflow amount C₀necessary for cooling the non fuel bearing components when (d/D) is usedas the parameter. More specifically, in the range of (C/C₀)≦1, thecoolant inflow amount C does not exceed the necessary coolant inflowamount C₀. In the range of (C/C₀)>1, the coolant inflow amount C exceedsthe necessary coolant inflow amount C₀.

As shown in FIG. 7, in the range of (d/D)>0.04, (C/C₀)>1 is established,and the coolant inflow amount C becomes larger than the necessarycoolant inflow amount C₀. In the range of (d/D)≦0.04, (C/C₀)≦1 isestablished, and the coolant inflow amount C does not exceed thenecessary coolant inflow amount C₀.

Hence, from the viewpoint of the cooling ability, the diameter d of thedrain hole 15 of the thimble screw 14 and the inner diameter D of thelower large-diameter portion of the guide thimble 2 must satisfyd>0.04D.

In the fuel assembly according to this embodiment, an improved guidethimble is employed, and the inner diameter D of the lowerlarge-diameter portion of the guide thimble 2 and the diameter d of thedrain hole 15 of the thimble screw 14 are adjusted to satisfy0.04D<d<0.08D.

Hence, the coolant can be sufficiently supplied also from the viewpointof assuring the cooling function of the non fuel bearing components.From the viewpoint of moderating the fall impact of the control rod aswell, the terminal velocity V of the control rod can be suppressed to beequal to or less than the fall velocity with which the fall impact ofthe control rod can be moderated. Therefore, flexural deformation of adashpot 20 can be prevented.

Second Embodiment

The second embodiment of the present invention will be described withreference to FIG. 8 and FIGS. 9A to 9C.

FIG. 8 is a view showing a state wherein a rotation preventive pin 17for a thimble screw 14 in a fuel assembly according to the presentinvention is built into the thimble screw 14.

FIGS. 9A, 9B, and 9C are views each showing a rotation preventive pinfor a thimble screw in the fuel assembly according to this embodiment.

As shown in FIG. 8, a shaft 23 of the thimble screw 14 has a guide hole24 as a hole extending from a spot facing hole 18 of a seat 16 to adrain hole 15 on the distal end side in the longitudinal direction ofthe thimble screw 14. With the rotation preventive pin 17 being mountedin the spot facing hole 18, during operation of the nuclear reactor, acoolant enters from the spot facing hole 18 of the seat 16 as shown inthe direction indicated by an arrow A and is drained from a drain holedistal end 25. If the control rod is dropped in the scram mode, thecoolant enters the drain hole 15 from the drain hole distal end 25 asshown in the direction of an arrow F and is drained from the spot facinghole 18 of the seat 16.

In the thimble screw 14 of the fuel assembly according to thisembodiment, the rotation preventive pin 17 has, at its upper side namelythe guide hole 24 side, a water receiving machined portion 26 formed ofa recess with an arcuate section, so it receives the flow of the coolantpassing through the guide hole 24 in the direction indicated by thearrow F in FIG. 8. The water receiving machined portion 26 increases thepressure drop of the coolant flowing in the direction of the arrow F.

FIGS. 8 and 9C show an arcuate machined portion 30 formed of a recesswith an arcuate section as a typical example of the rotation preventivepin 17 with the water receiving machined portion 26. Alternatively, thewater receiving machined portion 26 may be a V-shaped machined portion28 with a V-shaped section, as shown in FIG. 9A, or a flat machinedportion 29, as shown in FIG. 9B. The water receiving machined portion 26can have any shape as far as it can increase the pressure drop in thedirection of the arrow F against the flow of the coolant passing throughthe guide hole 24 in the direction of the arrow F, when compared to aconventional case wherein a rotation preventive pin without a waterreceiving machined portion 26 is used.

With the structure of the conventional rotation preventive pin, theratio of the pressure drop coefficient for the flow of the coolantentering from the spot facing hole 18 of the seat 16 and draining fromthe drain hole distal end 25 as shown in the direction of the arrow A,to the pressure drop coefficient of the flow of the coolant enteringfrom the drain hole distal end 25 and draining from the spot facing hole18 of the seat 16 as shown in the direction of the arrow F, in theopposite manner, is almost 1:1.

The thimble screw 14 of the fuel assembly according to this embodimenthas the rotation preventive pin 17 with the above arrangement. Thus, theratio of the pressure drop coefficient of the flow of the coolantentering from the spot facing hole 18 of the seat 16 and draining fromthe drain hole distal end 25 as shown in the direction of the arrow A,to the pressure drop coefficient of the flow of the coolant enteringfrom the drain hole distal end 25 and draining from the spot facing hole18 of the seat 16 as shown in the direction of the arrow F, in theopposite manner, can be raised to the range of 1:2 to 1:3.

With the thimble screw 14 of the fuel assembly according to thisembodiment, when the rotation preventive pin 17 with the shape asdescribed above is used, the thimble screw 14 can also serve as a diode.Thus, while the non fuel bearing components have the same coolingability as that of the conventional case, which is caused by the flow ofthe coolant in the direction of the arrow A, the decelerating effect ofthe control rod can be improved by the increase in fluid resistanceagainst the flow in the direction of the arrow F.

Third Embodiment

The third embodiment of the present invention will be described withreference to FIGS. 10 and 11.

FIGS. 10 and 11 are views each showing a thimble screw in a fuelassembly according to this embodiment.

In a thimble screw 14 for the fuel assembly according to thisembodiment, a guide hole 24 is formed in the thimble screw 14 on adistal end side 31, and a drain hole 15 is arranged in the thimble screw14 on a seat 16 side.

Referring to FIG. 10, a shaft 23 of the thimble screw 14 has the drainhole 15 extending between the guide hole 24 and a spot facing hole 18 ofthe seat 16. The opening area of the drain hole 15 is set smaller thanthe opening area of the guide hole 24 or the opening area of the spotfacing hole 18. With a rotation preventive pin 17 being mounted in thespot facing hole 18, during operation of the nuclear reactor, a coolantenters from the spot facing hole 18 of the seat 16 in the direction ofan arrow A in FIG. 10 and is drained from a guide hole distal end 32.

When a control rod is dropped in the scram mode, the coolant flows inthe direction of an arrow F and enters from the guide hole distal end 32to flow through the spot facing hole 18 of the seat 16. After passingthrough the drain hole 15, the coolant forms a jet as the flow path areais abruptly increased by the spot facing hole 18 of the seat 16, andjets out toward the rotation preventive pin 17.

With this arrangement, when the control rod is dropped in the scram modeand the coolant enters from the distal end side 31 in the direction ofthe arrow F, the rotation preventive pin 17 functions strongly as thefluid resistance against the jet. Thus, the pressure drop against theflow of the coolant in the direction of the arrow F can be increased,and the decelerating effect of the control rod can be improved.

In the thimble screw 14 of the fuel assembly shown in FIG. 11, therotation preventive pin 17 with the water receiving machined portion 26in the thimble screw 14 shown in FIG. 8 and FIG. 9A, 9B, or 9C iscombined with the thimble screw 14 with the arrangement shown in FIG.10. This rotation preventive pin 17 has a water receiving machinedportion 26, in the same manner as the rotation preventive pin 17 of thesecond embodiment.

Since the thimble screw of the fuel assembly according to thisembodiment has the above arrangement, the coolant enters from the guidehole distal end 32 in the direction of the arrow F shown in FIG. 10 andis discharged in the form of a jet from the drain hole 15 toward therotation preventive pin 17. As the rotation preventive pin 17 stronglyfunctions as a fluid resistance against the jet, the pressure drop forthe flow of the coolant in the direction of the arrow F can beincreased, and the decelerating effect of the control rod can beimproved. Meanwhile, the rotation preventive pin 17 does not influencethe flow rate resistance of the coolant in the direction of the arrow A.Thus, the coolant flow rate is assured, and the cooling ability of thenon fuel bearing components can maintain the same effect as that of theconventional case.

When the rotation preventive pin 17 with the water receiving machinedportion 26 is combined with a thimble screw in which the opening area ofthe drain hole 15 is set smaller than the opening area of the guide hole24 or the opening area of the spot facing hole 18, as in the thimblescrew 14 of the fuel assembly shown in FIG. 11, the difference inpressure drop of the flow in the direction of the arrow A or F furtherincreases. Therefore, a thimble screw for a fuel assembly with a betterdiode performance can be provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1.-3. (canceled)
 4. A fuel assembly comprising: a bottom nozzleconfigured to be disposed on a lower plate of a nuclear reactor; a topnozzle including a hold down spring configured to urge the bottom nozzletoward the lower plate; a plurality of control rod guide tubesconfigured to guide control rods, passed through the top nozzle towardthe lower plate; and a thimble screw locked to the bottom nozzle at aseat with a rotation preventive pin to connect the control rod guidetubes to the bottom nozzle, the thimble screw comprising a drain holeextending in a longitudinal direction from a spot facing hole of theseat to a distal end and configured to receive coolant supplied into thedrain hole from the spot facing hole toward the distal end while thenuclear reactor operates and to receive coolant supplied into the drainhole from the distal end toward the spot facing hole during a scrammode, and a coolant collision portion at a drain hole side of therotation preventing pin against which the coolant flowing from thedistal end toward the spot facing hole collides in order to increasepressure drop of the coolant during the scram mode, wherein a collisionsurface of the coolant collision portion against which the coolantcollides forms a flat surface.
 5. (canceled)
 6. A fuel assembly,comprising: a bottom nozzle configured to be disposed on a lower plateof a nuclear reactor; a top nozzle including a hold down springconfigured to urge the bottom nozzle toward the lower plate; a pluralityof control rod guide tubes configured to guide control rods, passedthrough the top nozzle toward the lower plate; and a thimble screwlocked to the bottom nozzle at a seat with a rotation preventive pin toconnect the control rod guide tubes to the bottom nozzle, the thimblescrew comprising a drain hole extending in a longitudinal direction froma spot facing hole of the seat to a distal end and configured to receivecoolant supplied into the drain hole from the spot facing hole towardthe distal end while the nuclear reactor operates and to receive coolantsupplied into the drain hole from the distal end toward the spot facinghole during a scram mode, the drain hole having a first large innerdiameter portion at a distal end side, a second large inner diameterportion at a seat side, and a small inner diameter portion between thefirst and the second large inner diameter portions, the spot facing holeis disposed on the seat side, thereby flow rate resistance of thecoolant is not influenced while the nuclear reactor operates, andpressure drop for the flow rate of the coolant is increased anddecelerating effect of the control rods is improved during the scrammode.
 7. A fuel assembly according to claim 6, wherein the thimble screwcomprises a coolant collision portion provided at the seat, againstwhich the coolant flowing from the distal end toward the spot facinghole collides to increase pressure drop of the coolant during the scrammode, wherein a collision surface of the coolant collision portionagainst which the coolant collides forms a flat surface.
 8. (canceled)9. A fuel assembly, comprising: a bottom nozzle configured to bedisposed on a lower plate of a nuclear reactor; a top nozzle including ahold down spring configured to urge the bottom nozzle toward the lowerplate; a plurality of control rod guide tubes configured to guidecontrol rods, passed through the top nozzle toward the lower plate; anda thimble screw locked to the bottom nozzle at a seat with a rotationpreventive pin to connect the control rod guide tubes to the bottomnozzle, the thimble screw comprising a drain hole extending in alongitudinal direction from a spot facing hole of the seat to a distalend and configured to receive coolant supplied into the drain hole fromthe spot facing hole toward the distal end while the nuclear reactoroperates and to receive coolant supplied into the drain hole from thedistal end toward the spot facing hole during a scram mode, the drainhole having a large inner diameter portion at a distal end side and asmall inner diameter portion at a seat side, the spot facing holedisposed on the seat side, wherein the thimble screw comprises a coolantcollision portion provided at the seat, against which the coolantflowing from the distal end toward the spot facing hole collides toincrease pressure drop of the coolant during the scram mode, wherein acollision surface of the coolant collision portion against which thecoolant collides forms a flat surface. 10.-22. (canceled)